Closed-circuit plethysmography and inert gas equilibration analysis were used to measure the volumes of gas in the respiratory system and plumage at the end of voluntary dives in unrestrained lesser scaup (Aythya affinis). Total (respiratory plus plumage) gas volumes were measured by helium dilution and estimated from body mass, body tissue density and buoyant force. These two techniques yielded results that differed by only 2.1 +/- 1.5%. Buoyancy decreased from a maximum of 3.46 +/- 0.16 N at immersion to a minimum of 2.65 +/- 0.16 N at 1.5 m depth at the end of the feeding phase of voluntary dives. At 0.193 +/- 0.013 L BTPS, the respiratory system contributed 52% of the initial buoyancy and 65% of the minimum value. The increasing relative influence of the respiratory system on buoyancy was due to the loss of 47 +/- 5% of the air in the plumage layer during the dive. These data differ significantly from estimates based on restrained ducks, and this has implications for modelling of mechanical costs of diving, oxygen storage capacity and thermal insulation in foraging ducks.